Ink jet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital data signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. In another process, known as continuous ink jet, a continuous stream of droplets is formed and individual droplets are differentially deflected from the stream in an image-wise manner onto the surface of the image-recording element, while un-imaged droplets are caught and returned to an ink sump. Ink jet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
The ink compositions known in the art of inkjet printing may be aqueous- or solvent-based, and in a liquid, solid or gel state at room temperature and pressure. Aqueous-based ink compositions are preferred because they are more environmentally friendly as compared to solvent-based inks, and most printheads are designed for use with aqueous-based inks.
The ink composition may be colored with pigments, dyes, polymeric dyes, loaded-dye/latex particles, or any other types of colorants, or combinations thereof. Pigment-based ink compositions are advantageous because such inks render printed images giving comparable optical densities with better resistance to light and ozone image degradation as compared to printed images made from other types of colorants. The colorant in the ink composition may be yellow, magenta, cyan, black, gray, red, violet, blue, green, orange, brown, etc. These inks may further contain polymeric binders.
Although numerous ink compositions are known in the art of inkjet printing, several key challenges remain. One challenge is to obtain the highest possible image quality on a variety of inkjet receivers. It is desirable to obtain the highest optical density for a given amount of ink applied to an inkjet receiver, especially on plain papers.
In addition to the necessity for an ink to have a good optical density when printed on plain paper, there are a number of constraints on the physical properties of an inkjet ink so that it can function effectively in an inkjet printer and make a lasting image. These properties include viscosity and rheology, ink physical stability, redispersibility of dried ink for circulating systems, surface tension and wetting, and jetting performance including drop formation stability, satellite suppression, print window, latency, and repeated firability. It is also important that inks dry fast on the paper, do not repel one another, and absorb into the substrate without bleeding when over printed with different colors. The dried inks need to have good image permanence including fade and scratch resistance.
A further challenge for inks comprising both pigments and polymeric binders is managing their ability to function in the printer system. Ink must properly wet the felt employed in an inkjet cartridge to regulate pressure in a printhead so that flow of ink from the cartridge and through the printhead occurs only when desired. Management of the surface tension of inks is also required to enable delivery of ink through a printhead in addition to aiding wetting of the surface of the substrate to which the ink is applied. Inkjet printheads and ink cartridges employ wicking materials to prevent leakage of ink through the printhead nozzles when not in use. The ink must have sufficient affinity for the wicking material (felt) to allow the ink to be drawn into the delivery system from the ink tank.
U.S. Pat. No. 7,878,643 discloses dye-based ink formulations with preferable ranges for dynamic surface tension at 50 ms and at 500 ms thetimes, as determined by maximum bubble pressure method (referred to herein as MBP nominal surface age, or MBP age), with a difference between these dynamic surface tensions of 7 mN/m or more.
U.S. Pat. No. 7,862,653 teaches that it is desirable to have the dynamic surface tension of the ink to be at least 49 mN/m or more at a lifetime of 50 milliseconds as determined by MBP method for improved optical density. The patent further discloses that this ink preferably have a difference between the dynamic surface tension at a lifetime of 50 milliseconds (MBP method) and the dynamic surface tension at a lifetime of 5,000 milliseconds (MBP method) of 15 mN/m or more to obtain improvements in both optical density and fixing ability. The dynamic surface tension difference in this patent is accomplished through the use of polyoxyethylene alkyl ether surfactants. Surface tensions at these short times, however, may not dictate the optical density performance, and thus may be an insufficient criterion to select the best inks.
Controlling surface tension using fluorinated surfactants has been employed in various ink formulations in the art, frequently in combination with the use of other classes of surfactants where either or both surfactants are at relatively high concentrations in the inks, such as in U.S. Patent Application 2007/0120928. In some cases, the fluorinated surfactants are disclosed in formulations without other surfactants, such as in the following publications: U.S. Patent Application 2008/0049086, U.S. Patent Application 2006/0116439, U.S. Pat. No. 7,478,902, U.S. Pat. No. 7,622,513, and U.S. Pat. No. 7,696,262. These publications, however, typically disclose the use of fluorosurfactants at relatively high levels (typically greater than 0.1 wt %), in combination with surface active co-solvents such as 1,2-hexanediol.
Generally the use of very low concentration surfactants yields high static surface tension inks which have been deemed undesirable for ink formulation and ignored in the art because of the practical challenges in delivering the ink from the ink cartridge to the ink printhead. Typically “felt” wicking materials will not allow a smooth stream of ink to transfer to the printhead if the surface tension of the ink is not sufficiently low. High surface tension inks are also slow to penetrate the paper which will yield improved optical density if allowed to dry unperturbed, but are typically prone to smear and thus impractical.